A self-propulsion gas system generally comprises a gas cylinder or tank under high pressure arranged in the trunk of a vehicle, although away from the engine, a pipeline from the cylinder or reservoir to the engine, and valve and pressure reducing devices arranged along the pipeline for controlling the gas flow directed towards the engine. The pressure of the gas in the cylinder, if methane, may be, e.g., about 220 bar, and the pressure of the gas to the engine, depending on the feed system of the latter, may be about 0.1-1.0, on average 0.5 bar, or about 9 bar.